Electric Motors & Generators

The Motor Effect

Before discussing the motor effect it is important to gain an understanding in magnets and magnetic field.

Magnets are materials normally with iron in them that produce a magnetic field. They attract other pieces of iron bought close to them with a magnetic force.

The region around a magnet where a magnetic effect can be felt is called the magnetic field.

A magnet has two poles;

North seeking pole or North Pole

South seeking pole or south Pole

The magnetic field is strongest at its poles. The field around a magnet can be represented by lines with arrows on them. The arrows show the direction of the lines of force. Each field line starts at the North Pole and finishes at the South Pole.

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Magnets affect a wire conducting electricity; this is because an electric current in a wire produces a magnetic field. If a wire carrying a current is placed in a magnetic field of a magnet it will experience a force due to the interaction between the magnetic field of the magnetic and the magnetic field of the current in the wire.

This force the electric wire experiences is called the motor effect and only happens when the wire is carrying electricity.

The direction of the lines of force around a wire carry a current can be determined using the Right-hand Grip Rule. If you were to image gripping a wire carrying a current so that your right thumb pointed in the same direction as the flow of electrical current then the fingers of your right hand curl in the direction of the magnetic field lines.

Right Hand Grip Rule

Fleming’s Left Hand Rule

The direction of movement of a current carrying wire in a magnetic field can be determined using Fleming’s Left Hand Rule.

By arranging your left hand as shown in the image above, the first finger points in the direction of the magnetic field (from North to South).

The hand is then rotated until the second finger points in the direction of the current (remember conventional current is from positive to negative).

The thumb then points in the direction of the movement of the wire.

The summary below aids in memorising the rule.

First Finger

=

Field (magnetic from North to South)

seCond Finger

=

Current (conventional current from +ive to –ive)

thuMb

=

Movement of the wire

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A Simple Electric Motor

An electrical motor is a device that converts electrical energy to mechanical energy. It works on the principle of the interactions between the magnetic fields of a permanent magnet and the field generated around a coil conducting electricity. The attractive and repulsive forces between the magnet and the coil create rotational motion.

A simple electric motor consists of the following parts.

A permanent magnet

Armature or rotor
This consists of a thin copper wire coiled around an iron core, hence when electric current flows it acts as an electromagnet. In the case of a simple motor this is a wire loop.

Commutator
A Commutator is a copper ring split in two halves. In a simple electric motor each half is connected to the ends of the wire loop. In practise they are connected to the axle.

Brushes
The brushes connect the wire loop or armature to the power supply

Axle
In electric motors the commutator is attached to the axle. The axle transfers the rotational motion.

Power supply (battery)

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Improving a Motor

An electric motor can be made powerful by the following;

By increasing the number of turns that are wound on the coil. In the case of the wire loop in the animation this would mean winding to form two loops.

By winding the electrical wire around a soft iron core so that the magnetic field is stronger.

By replacing the permanent magnet with a electromagnet which can gice a stronger magnetic field.

By winding extras coils around the core. Similar to having two separate wire loops in the animation above however this would mean splitting the commutator into 4 parts.